This project will be concerned with a study of two bacteriocidal substances which are lethal for Streptococcus pyogenes. One of these is produced by S. sanguis (group H) and the other by S. mutans. Our major effort will be to determine the properties and characteristics of these streptocins with emphasis on the mechanism of action on the streptococcal cell. A determination will also be made of their effect on the progress of group A infection in animals following intraperitoneal or other means of inoculation with virulent strains of S. pyogenes, and their effect on the action of polymorphonuclear leucocytes on the streptococcal cells under these conditions.